Hybridization

hybridization

The production of viable hybrid somatic cells following experimentally induced cell fusion.

(genetics)

Production of a hybrid by pairing complementary ribonucleic acid and deoxyribonucleic acid (DNA) strands.

Production of a hybrid by pairing complementary DNA single strands.

(physical chemistry)

The mixing together on the same atom of two or more orbitals that have similar energies, forming a hybrid orbital.

Hybridization

the crossing of organisms differing in heredity—that is, in one or more pairs of alleles (conditions of genes) and consequently in one or more pairs of traits and properties. The crossing of individuals belonging to different species or even to less closely related taxonomic categories is called distant hybridization. The crossing of subspecies, varieties, or breeds is called intraspecific hybridization. The process of hybridization—especially natural hybridization— was observed in very ancient times. Hybrid animals (for example, mules) existed as early as the second millennium B.C.. The possibility of producing hybrids artificially was first suggested by the German scientist R. Camerarius (1694). The first to carry out artificial hybridization was the English horticulturist T. Fairchild, who crossed different species of pinks in 1717. The founder of teaching on sex and hybridization in plants is thought to be J. G. Kolreuter, who obtained hybrids of two tobacco species—Nicotiana paniculata and N. rustica (1760). G. Mendel’s experiments on the hybridization of peas laid the scientific foundation of genetics, and Darwin performed an enormous number of experiments on hybridization.

The essence of hybridization is the fusion during fertilization of genotypically different sex cells and the development from the zygote of a new organism that combines the hereditary disposition of the parents. Copulation in unicellular organisms is also included among the phenomena of hybridization. The first generation of hybrids is often characterized by heterosis, which is manifested in better capacity for adaptation and greater fertility and viability of organisms. Hybridization as well as mutations are the main sources of hereditary variation, which is one of the main factors in evolution.

In natural hybridization and in artificial hybridization that is carried out by man for breeding and other purposes, flowers of the maternal form are pollinated with pollen from another species (variety) of plant, or animals of different species (subspecies, breeds) are mated. The sexual process guarantees the combining of genomes and results in the union of the nuclei of gametes—karyogamy. Therefore, it is impossible to obtain so-called vegetative hybrids. The “vegetative” hybrids described by some investigators are simply tissue chimeras.

In livestock breeding, intraspecific hybridization is a method of industrial breeding by which individuals of different breeds or strains are mated. Distant hybridization in animals is the obtaining of hybrids between varieties, species, and genera—for example, the crossing of fine-wooled sheep and Pamir argalis or cattle and zebus. This is difficult to accomplish, and the hybrids are generally sterile.

In 1935 the Soviet geneticist G. D. Karpechenko made a distinction between congruent crossings, or hybridization, and incongruent crossings in plants. Congruent crossings are intraspecific and sometimes interspecific crossings in which parental pairs with homologous chromosomes are crossed, and the offspring are fertile. Incongruent crossings are generally distant crossings—that is, crossings of two individuals with structurally incompatible chromosomes and differences in the chromosome number or cytoplasm. The offspring are partly or completely sterile and the nature of the segregation is complex.

Crossings may be direct or reciprocal. For example, the hybrids ♂ A × ♀B and ♀B + ♂ A are reciprocal. If a hybridis crossed with one of the parental forms, the crossing is called a backcross. A testcross involves backcrossing a hybrid with a parent that is recessive for the trait under study. This is done to establish the hybrid’s heterozygosity, linkage groups, or the frequency of crossing over between linked genes. Repeated backcrossing of a hybrid with one of the parents is called saturation. It is used to introduce into genotype A the traits of genotype B or to transfer the genome to the cytoplasm of another variety, subspecies, or species. There are also complex crossings called convergent crossings. First the parental varieties are crossed in pairs. The hybrids are then crossed with each other, and the newly produced hybrids are crossed with each other. In such cases individual hybrids often have valuable combinations of properties and traits.

Hybridization is widely used in breeding. Depending on the purpose of hybridization, there may be “combination” breeding to combine the desirable traits of the parental forms and “transgressive” breeding to obtain and select genotypes that are superior to parents in the bred trait.

Plant growing. The most common method of breeding plants is the hybridization of forms or varieties within a single species. This was the method used to create the majority of modern varieties of agricultural crops. Distant hybridization is a more complicated and laborious method of obtaining hybrids. The main obstacle to obtaining distant hybrids is the incompatibility of the gametes of the pairs to be crossed and the sterility of the hybrids of the first and succeeding generations. The use of polyploidy and backcrossing makes it possible to overcome the noncrossability of the pairs and sterility of the hybrids. Other methods are also used, including mixture of pollen, preliminary vegetative rapprochement, and application of a gibberellin solution to the stigma of the pistil. The degree of sterility of distant hybrids depends on the phylogenetic relationship of the species crossed and the presence of homologous chromosomes or genomes in the gametes of the first generation hybrid. In case of complete asynapsis—that is, the absence of homologous chromosomes—the hybrids are sterile (for example, wheat-wild rye, wheat-rye, rye-wheatgrass, and many other hybrids).The technique of hybridization varies with the crop. To obtain corn hybrids, the varieties (strains) intended for hybridization are sown in alternating rows and the tassels are removed from the maternal plants several days before they flower. In cross-pollinated crops—for example, rye—the flowers of the maternal plants are castrated. The castrated ears are covered with caps and flowering paternal ears, which are placed in bottles of water suspended from special poles. Fruit plants are castrated one to three days before the buds open. The remaining female flowers are isolated by a two-layer gauze bag. Previously gathered pollen is applied to the stigmata of the pistils. The fertilized flowers are isolated again. Hybrid seeds—especially those produced by distant hybridization—are usually sickly and underdeveloped, and it is difficult to grow a hybrid plant from them. The best method involves removing the embryos from the hybrid seeds and placing them on an artificial nutrient medium.Distant hybridization is used to produce plant forms with valuable yield and resistance to fungus diseases and pests. Interspecific sunflower hybrids produced by Academician V. S. Pustovoit and G. V. Pustovoit have seeds that are up to 55 percent oil and are distinguished by group immunity to diseases and parasites. The Bezostaia 1 and other wheats, which were obtained by Academician P. P. Luk’ianenka and which are characterized by high yields, adaptability, and other valuable traits, are examples of the successful hybridization of geographically distant forms. By crossing cultivated tobacco with wild species M. F. Ternovskii created a variety of tobacco of the highest quality that is immune to tobacco mosaic, powdery mildew, and tobacco blue mold. Valuable results were obtained by hybridizing cultivated potato varieties with wild species. B. S. Moshkov crossed radish and cabbage to produce a hybrid whose foliage and underground parts are used as lettuce and radish, respectively. Academician N. V. Tsitsin hybridized cultivated plants (wheat, rye, and barley) with five species of wild Agropyrum and three species of Elymus.

Animal husbandry. In zootechny, a distinction is made between hybridization proper and interbreeding of animals whose offspring are called crossbreeds. Crossbreeds are easily crossed and produce offspring. Hybrid animals are often difficult to obtain, and the resulting hybrids are frequently partly or completely sterile, which makes it difficult or impossible to breed them any further. The difficulties of hybridization result from many factors. Structural differences between the sexual organs of animals of different species make copulation difficult, and the male of one species may fail to respond sexually to the female of another species. These two difficulties have been eliminated by the use of artificial insemination in hybridization. The spermatozoa of one species may die in the genital tract of the female of another species. The spermatozoa may not react to the egg cell of females of another species, thus making fertilization impossible. The zygote may die, disturbances in fetal development may result in the birth of monsters, and hybrids may be completely or partly sterile. As to overcoming the uncrossability of different species caused by other factors, there have been only a few experiments, and they were inadequately performed or involved methodological errors. In case of complete sterility, both sexes are incapable of producing hybrids. If sterility is partial, one sex is sterile— usually the male among mammals. When males are sterile, further work with hybrids is done by crossing hybrid females with males of one of the original species. This frequently results in the loss of valuable traits in the hybrids. The phenomenon of heterosis (increased vigor) often occurs in hybrid offspring, and it is more pronounced in them than in crossbreeds.Hybrids of horses and asses (mules, hinnies), horses and zebras (zebroids), one-humped and double-humped camels, and yaks and zebus with cattle have been obtained since ancient times. Hybrid animals are generally superior to the parental forms in many economically valuable qualities, including work capacity, endurance, and productivity. In the United States, crossing Brahman bulls (India) with shorthorn cows produced the specialized Santa Gertrudis beef cattle, which have been imported into the USSR. In Askania-Nova, crossing red steppe cattle with zebus produced zebu-like cattle distinguished by the higher fat content of their milk and by a greater resistance to piroplasmosis than red steppe cattle. Hybrids have been produced by crossing cattle with gayals, aurochs, and bison and by crossing aurochs with bison and bison with yaks, zebus, and gayals. Attempts to cross water buffalo with cattle have been unsuccessful.In swine breeding domestic swine are crossed with wild boars to strengthen the build of cultivated breeds and improve their capacity to adapt to local conditions. In the Kazakh SSR, crossing wild Middle Asian swine with the large white and Kemerovo breeds produced a new breed of meat-lard swine—the Kazakh hybrid, which is well adapted to the climatic and fodder conditions prevailing in southeastern Kazakhstan. In sheep breeding, crossing domestic sheep with wild mouflon and Pamir argali rams produced new breeds—mountain merino and Kazakh argali-merino. Sheep have not yet been successfully hybridized with goats. In poultry farming, hybridization produced interesting hybrids of domestic hens with peacocks, roosters with turkey hens and guinea hens, peacocks with guinea hens, and musk ducks with domestic drakes.Valuable hybrids have been obtained in fish breeding. Hardy intraspecific hybrids of scaly and mirror carp with Amur carp have been bred for the fish farms of the USSR. These hybrids can develop normally in waters of the northern regions, where cultivated carp breeds die during their first winter. Intergeneric hybrids of carp and crucian carp have been produced. They are comparable to carp in food value, and they inherit the considerable hardiness of crucian carp. Whitefish are increasingly hybridized for pond fish culture. The hybridization of sturgeon is practicable. Belugas are crossed with sterlets and sturgeon and sturgeon with sterlets and other breeds that are not widely found in pond culture. As in plant growing, intergeneric crossing is called hybridization in silkworm breeding. Therefore, for example, the offspring resulting from the crossing of the Belokonnaia 1 with the Belokonnaia 2 or the SANIISh 8 with the SANIISh 9 breed are considered hybrids.The experiments and practical accomplishments in the field of animal hybridization have great theoretical scientific and economic significance.

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